# RE: [LEAPSECS] name the equinox contest on now

From: Seeds, Glen <Glen.Seeds_at_Cognos.COM>
Date: Thu, 29 Jan 2004 16:20:10 -0500

I agree. I personally am struggling with a fairly fundamental gap in
undesrstanding the stuff at this site. One of the documents there
(http://syrte.obspm.fr/iauWGnfa/Annex1.doc) gives some definitions that are
particularly germane to this group:

Universal Time (UT): a measure of time that conforms, within a close
approximation, to
the mean diurnal motion of the Sun and serves as the basis of all civil
timekeeping. It is
determined from observations of the diurnal motions of the stars. It has
several realizations: UT0, UT1, and UTC.

UT0: the Universal Time as determined directly from observations; it
is slightly dependent on the place of observation.

UT1: time scale deduced from the UT0 corrected for the shift in
longitude of the observing station caused by polar motion. It is
proportional to the Earth Rotation Angle (ERA), expressing the angle of
Earth's rotation about the CIP axis. It was related by a mathematical
formula to the sidereal time in the moving reference frame.

Elsewhere there are statements that UTC is maintained as a close
approximation of UT1, thereby relating it to this discussion.

The problem I have is this: All variants of UT are supposed to be a close
approximation of "the mean diurnal motion of the Sun". That makes perfect
sense to me. Yet there is nothing in the statements above of how they are
measured that seems to relate the earth to the *Sun*; rather, they all
relate the earth to the [distant] *stars*. Can someone please explain how
this can possibly work?

/glen

-----Original Message-----
From: Leap Seconds Issues [mailto:LEAPSECS_at_ROM.USNO.NAVY.MIL]On Behalf
Of Steve Allen
Sent: January 28, 2004 7:14 PM
To: LEAPSECS_at_ROM.USNO.NAVY.MIL
Subject: [LEAPSECS] name the equinox contest on now

As noted before in LEAPSECS, at the beginning of 2003 the IAU
effectively abolished the Vernal Equinox as we knew it, and new
formulae came into use for determining UT1, and thus leap seconds.

While the new paradigm of celestial coordinates is rigorously
defined in terms of mathematics, it is lacking in a common
terminology. There has been an IAU Working Group on Nomenclature
for Fundamental Astronomy, and they are currently fielding a
questionnaire on what names should be applied to the new concepts.

The working group is at
http://syrte.obspm.fr/iauWGnfa/

and the questionnaire at
http://syrte.obspm.fr/iauWGnfa/NFAQuest.html

The responses are wanted this week, but the fact is that it is
difficult to make sense of the proposals without familiarity
with the past 20 years of literature on coordinate systems.

```--
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Received on Thu Jan 29 2004 - 13:20:46 PST

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